This invention relates to a non-aqueous electrolyte cell using metallic lithium, a lithium alloy, or an intercalation compound formed between lithium and carbon or lithium and metal oxide as the negative electrode and organic electrolyte, capable of being used for long periods and/or being stored in a high-temperature environment.
An organic electrolyte used conventionally in a non-aqueous electrolyte cell has been prepared by dissolving lithium salt as a solute into an organic solvent. This organic electrolyte has been selected because of its high stability in the presence of active metallic lithium and its low melting point.
Because the non-aqueous electrolyte cell has excellent low temperature characteristics, wide operable temperature range, and superior long-period storage characteristics, it has been extensively used as a main power source for various applications such as consumer-use electronic watches, electronic note-books, auto-focus cameras, etc. However, the use of such cells as memory back-up power-supplies in various electronic circuits is now expanding very rapidly. The typical cell systems for these applications can be formed of lithium-graphite fluoride (Li/(CF).sub.n) and lithium--manganese dioxide (Li/MnO.sub.2) systems.
A typical longitudinal sectional view of a conventional Li/(CF).sub.n coin-type nonaqueous electrolyte lithium cell is shown in FIG. 1. In FIG. 1, metallic lithium acting as a negative electrode 2 is compressed in one body to the inner surface of cover case 1. Cover case 1 is made of stainless steel and acts as a negative terminal. Gasket 6 is mated on the flange of cover case 1 and molded positive electrode 3 consisting essentially of (CF).sub.n active material is pressed on a current collector. The current collector is made of titanium lath and connected on the inner surface of bottom case 5. Bottom case 5 is made of stainless steel (SUS 304; containing Cr 15%, Ni 8%) and acts as a positive terminal. Negative electrode 2 and positive electrode 3 are separated by a mat-like separator 4.
The organic electrolyte is impregnated and held within an opening formed in positive electrode 3 and separator 4. The flange of metal case 5 is curled and pressed on the gasket placed between cover case 1 and bottom case 5 in order to seal case 5.
Conventionally, sealant layer 8 consisting essentially of blown-asphalt is disposed in advance at least on a surface region of gasket 6, which is in contact with cover case 1 to improve the sealing between cover 1 and case 5. Conventional Li/(CF).sub.n system non-aqueous electrolyte lithium cells use a separator made of a polypropylene (PP) non-woven cloth and a gasket consisting essentially of polypropylene (PP) resin. An organic electrolyte prepared by dissolving a solute of lithium fluoborate (LiBF.sub.4) into a high-boiling point solvent such as .gamma.-butylolactone (BL), or into a mixed solvent such as of BL and a low boiling point solvent, such as 1,2-dimethoxyethane (DME), where the solvents are mixed at a ratio yielding a concentration of 1.0 mol/l.
The thermal decomposition temperature of graphite fluoborate employed as a positive active material in the Li/(CF).sub.n system cell is within a temperature range from 320.degree. to 420.degree. C. and produces no melting deformation when it is used as a negative electrode up to the melting point of metallic lithium which is 180.54.degree. C.
Since the thermal decomposition temperature of lithium fluoride (LiF) (which is a discharge product) is 848.degree. C., the Li/(CF).sub.n system cell is primarily a thermally stable cell. Thus, long storage periods and/or use of conventional Li/(CF).sub.n system cells for more than 10 years has been possible within a temperature range of -40.degree. to +60.degree. C.
Because the electrolyte cell is air- and liquid-tight, internal cell-elements such as the electrolyte are prevented from leaking and external elements such as air and moisture are prevented from entering into the cell. The electrolyte cell can be sealed by applying a sealant such as a plastic gasket having high electrolyte resistance and electrical insulation characteristics between the cover case 1 acting as a positive terminal and the bottom case 5 acting as a negative terminal regardless of the type of cells. The cells which may be used include coin-type, button-type, wafer-type, and cylindrical-type cells. In the case of conventional Li/(CF).sub.n system cells, a long storage period and/or use of more than 10 years is possible if the cell is within a temperature range of -40.degree. to +60.degree. C.
With the recent rapid advancement of electronics technologies, the use of cells for memory back-up has expanded very rapidly. Accordingly, the cell has to withstand more rigorous conditions such as high-temperatures and high humidity conditions produced within the automobile engine compartment or environments in which outdoor industrial equipment is operated.
Moreover, no deterioration of the cell characteristics is permissible when the cell is mounted on a printed circuit board and combined with other electronic components which are exposed to a rigorous high temperature situation such as reflow soldering.
As a non-aqueous electrolyte other than a Li/(CF).sub.n system, a solvent obtained by mixing propylene carbonate (PC) having a high-boiling point and 1,2-dimethoxyethane (DME) having a low boiling point is used extensively. The purpose of the addition of low boiling point solvent is to lower the viscosity of electrolyte and to improve the high-rate discharge characteristics, including the pulse discharge characteristics.
When a conventional non-aqueous electrolyte cell is subjected to a long storage period and/or used in a high-temperature environment, or a thermal shock of wide temperature difference is applied to the cell, a minute gap may be produced between the cell container and the gasket. If vapor from the low boiling point solvent contained in the organic electrolyte leaks out from the cell, it may cause problems such as a change in the electrolyte composition. In an extreme case, leakage of organic electrolyte and intrusion of external air may form an oxide layer on the surface of the lithium negative electrode due to the moisture contained in the air and causea deterioration of cell characteristics such as an increase in internal resistance.
In order to solve these problems, the development of a cell having improved heat-resistance is essential. In order to accomplish this objective, extensive efforts to improve the heat resistance of organic cell-materials such as the gasket, sealant, separator, etc., including the composition of electrolyte, have been made. For example, U.S. Pat. No. 5,246,795 and Japanese Patent Publication Hei 5-58232 show proposals included in these efforts, but the practical effects of these efforts are not satisfactory.
Furthermore, Li/SOCl.sub.2 system cells employing thionyl chloride (SOCl.sub.2 ; a strong oxidant) and corrosive liquid as the positive active material were developed which had a hermetically sealed construction. This system used a separator made of a glass fiber non-woven cloth and a cover on which the positive terminal was sealed by laser welding it (glass to metal) to the top edge of negative polarity case.
However, in the Li/SOCl.sub.2 system cells, the separator between the positive and negative electrodes is a layer of lithium chloride (LiCl on the surface of lithium) formed by the contact of the lithium negative electrode to the SOCl.sub.2. The glass fiber non-woven cloth should be called a spacer rather than a separator because of the large pore size of non-woven cloth.
Thus, if used as a separator in other cell systems internal short-circuits could easily take place, so that it would be unsuitable for such use. Although the hermetical seal accomplished by the glass-to-metal seal is highly reliable, these spacers cannot be produced at a fast rate and the cost is high so that the extensive use of these in general purpose cells is considered impractical.